Thread: Crank Case Ventilation fully explained. (Turbocharged edition.)

I'd also like to discuss the necessity of this situation. I think it is a great idea to put more obstacles between the oil gasses/liquids coming out of the valvecover and the turbocharger's compressor. Obviously this situation will promote cleanliness of the intake ducting, a great thing.

But there is also a negative side to this that must be mentioned because I believe it has been overlooked. As you increase the length of the tube that extends from the valvecover to the compressor, you are making it more difficult for the compressor to pull a vacuum signal on the crank case during WOT. Imagine if the length was increased by infinity, or 999999" for example. The farther you get from the compressor inlet, the less pressure drop (vacuum signal) the valve cover will see.

I think the key here is to keep the plumbing as short as possible when adding a catch can like this, and if possible, to install the fitting at an angle conducive to the general flow of molecules.

also, can we stop calling the oil separator a "catch can" ? Because it does not catch anything, at least not permanently, nor does it need to be drained.
And I think calling it a catch can is confusing folks who are new to the idea of PCV.

The factory setup allows for pcv during boost?
The point of the PCV valve is to let gasses out of the VC and not into it. The ideal pcv valve is completely shut during boost.

Interesting thoughts kingtal0n, but I can't say I share your point of view on most of them.

I really need to finish this thread... The images need editing to include the PCV restrictor, some of the images need other modification, and there need to be more examples.

I am sorry but the point of PCV is to provide a suction, that is, a pressure below atmospheric, during ALL operating conditions. You are thinking of the PCV VALVE which has a 1-way check-valve to prevent boost from entering the intake manifold. That is just the NAME of the valve, not the POINT of the system.

The POINT of the PCV system is to pull a vacuum on the entire crankcase during all operating condition. that is why we run a vacuum pump on 2000horsepower blower engines. That is why people spend thousands of dollars to bring their 500 cubic inch supercharged engines under vacuum during boost. And that is why there are several references to WOT PCV in this thread.

Then you are not reading my post. I made the same exact points (thoughts) you did, I only added one small detail.

Here you said it:



This is absolutely true and exactly why I posted here to begin with, I thought you of all people would remember what you write. There really ISNT much vacuum there during WOT.... And the longer the plumbing becomes (i.e. with external catch can) the less sufficient that vacuum will be when it gets to the valve cover.


For example,
Testing done at idle, stock sr20det all original,
Vacuum @ intake tract before turbo during idle:
1. With stock intake plumbing, NO air filter (15" of tube from valvecover to inlet) - 8mmH2O
2. 20" of tube - 7 mmH2O
3. 25" of tube - 5mmH2O
4. 30" of tube - 3mmH2o
5. 30" of tube + catch can - < 1 mmH2O

Vacuum @ intake tract before turbo during idle:
1. With stock intake plumbing, PAPER air filter - 12mmH2O
2. 20" of tube 12mmH2O
3. 25" of tube 11mmH2O
4. 30" of tube 9mmH2O
5. 30" of tube + catch can - 4mmH2O

In my exmaple, longer plumbing means less vacuum signal. Same with the FPR, if you have a long T-connection on your FPR you get a weaker/sluggish signal. Everybody knows the FPR gets a dedicated signal line, as short as possible. Why not the same treatment for PCV?

Fine. Here are your points, and my thoughts on the matter.

1) Breather filter on valve cover is not "acceptable" in turbo applications

This argument basically boils down to semantics. Call it acceptable or not, whatever... People can make up their own mind once they've read the information in these threads. There is a reason this method is considered unacceptable on N/A engines but acceptable on turbo engines. It is trivial to "do it right" on the N/A engine, so this method is silly. However on a turbo engine you have to balance coating the intercooler with oil versus not, and balance how much oil you'll tolerate lowering the octane of your fuel, making this method a somewhat viable option for turbo folks.


2) The breather filter on the valve cover does not maintain the stock prevention of pressurization of the crank case.

Like hell it doesn't. I think you're putting more meaning into this than there really is. Without splitting hairs, part of what the stock setup tries to prevent is the crankcase pressure from going over atmospheric. The stock setup lets pressure in the crank case over atmospheric enter the intake stream, limiting pressure in the crank case to atmospheric (or a tiny bit below). Putting a breather filter on the valve cover also limits pressure in the crank case to atmospheric as the pressure can vent to the atmosphere. That is all I mean by that component. You won't get 15 psi built up in the crank case, blowing out seals and wreaking all sorts of havoc. That's all.
Notice in that description that I spell out this method will NOT help promote ring seal and prevent blow-by (won't put a vacuum in the crank case, even the slightest amount). I also spell out that it will not help evacuate the crank case, again, because it doesn't put even the slightest vacuum to it.
But does it still prevent pressure build-up in the crank case? (Compare to someone sealing up the crankcase by capping every port off.) Yes, yes it does.


3) Shorter hoses and/or an overall less restrictive system is better.

Sure, that's true. But I don't think 2 feet of hose versus 5 feet of hose is anything to worry about. Really, I just don't. The size of the hose on our stock setups is HUGE for the amount of air it needs to flow. The reason for that is because there is a ~2.5mm restrictor in the line that makes the size of the hose almost irrelevant. As long as you keep the stock diameter hose throughout, you're going to be fine even with 10+ feet of it. This is a none issue IMO.


4) Install the bung to the intake pipe at an angle to aid scavenging.

Of course. This is great advice, and I could have sworn I mentioned it somewhere in the text. I didn't put it in the images because it was too difficult to fit everything in. When I do my setup, I will put the bung 45 degrees facing the direction of flow to aid scavenging. If I didn't specifically mention this as a really good idea, it was probably because I said this is going to be a primer and everything under the sun can't be spelled out here. I just didn't want to get into the minutia. I wanted to get people up to speed.


5) Stop calling them "catch cans".

I understand you're not directing this at me, since I pretty much universally call them oil separators... But here's the thing. There are such things as catch cans. They do catch oil, and they do need to be drained. I've seen them work and fill and be drained with my own eyes. There are self draining oil seperators (like the factory unit) which shouldn't be called catch cans, but I don't call them catch cans, so...


6) The PCV system provides a vacuum at all running conditions.

That would be ideal, however almost all factory setups fail to reach this goal. In turbo setups, they traditionally do their best at WOT to put a vacuum in the crankcase but they typically fail to do so. The best they can usually do is keep pressure down to a very minimum above atmospheric.

Did you have the a ~2.5mm restrictor in place between the intake tract and your test equipment?

Because the PCV setup has this restrictor with a tiny opening that makes anything you do with the huge hose almost irrelevant.

Very nice, thank you for reading and taking the time to address these points to narrow down the ideas.


#1 Semantics, personal preference. Yes people without any testing or data will have a personal preference on this matter. Some feel that having near atmospheric pressure in their crankcase during boost is "fine". Others, who have done the testing, who KNOW that it is NOT fine, spend a thousand dollars on a belt driven vacuum pump and build their engines accordingly to improve power output and engine efficiency during boost. Clearly, if a breather on the valvecover was just "fine" for all applications, then nobody would be spending thousands of dollars on vacuum pumps. Furthermore, the factory would not have the WOT PCV tube if it was "fine" to leave a breather on the valvecover! SO far, ALL the OEM and high dollar applications ALL use PCV during boost as a necessity. Only low dollar enthusiast projects, without data/testing, are the ones running breathers.




If the pressure is exiting the valvecover TO the atosphere during boost, then THERE MUST BE HIGHER PRESSURE IN THE CRANK CASE. Air molecules would not flow OUT of the valvecover during boost if there was NO pressurization of the crank case. True that pressure does not RISE enough to blow out seals. And YES You nailed it- it will not help prevent ring seal and prevent blow-by. ALL true and your line of thinking is pristine. However, the fact is, you STILL have a slight pressurization of the crank case, any way you slice it. Therefore, the breather filter, does NOT prevent pressurization of the crank case, by definition.




We need to do some testing, we cant just "think" something is better or worse. You started with TRUE because it is true, and then you "think" it isnt anything to worry about. You might be right, but true is true.

Also, I was actually not aware of any restrictor in the line. Have you seen this restrictor? Now I am madly curious.





A minor detail, but one often overlooked is all. I am just trying to help you finish off the thread once and for all with every minute detail. You appreciate me adding attention to detail to all your hard work right? I hope so, that is why I am taking the time to do this.




Certainly NOT directly towards you. Only that people are constantly confused by the word "catch can". They see the black box on the redtop and call it a catch can when it is clearly not catching anything. I correct them, pointing out that it does not catch anything, and then 5 minutes later somebody else is calling it a catch can again. I think noobs to the PCV need to see that black can on the redtop as an "oil separator" not a catch can. And all your diagrams have the word "catch can" on them.




that WOULD be ideal, and the closer to ideal we get, the better. Shorter hoses, proper angles. vacuum pumps. good oil seals. high quality piston rings and proper break in procedure. testing and data. All of it counts.

And again, that restrictor. My redtop doesnt have one. Ive never seen one. Are you sure it exists? You know my memory is a little foggy- I seem to recall something there now that you mentioned it. But no, my redtop doesnt have a restrictor I looked and looked.
you mean this?
http://www.supercars.net/gallery/132464/1737/946320.jpg

Not all of them look like that. Some are just a nice open tube. Mine is. I think Nissan revised it at some point.

As to the restrictor, I have several thoughts.

1. Even if there is a restrictor, we are only talking about a few inches of water. Not even 1" of Hg. The signal is so weak that whether a restrictor is present or not should not alter the pressure differential on either side.
2. All aftermarket turbocharger inlets contain an open pipe. Anything you fabricate will also likely be an open pipe. The restrictor is strictly an OEM thing, if it is ever present. So it does not always apply.

3. In the event of the OEM setup, you may have noticed that on a redtop, liquid oil is often found inside the rubber intake plumbing, and also often coating the compressor wheel. As you said already, it allow oil gasses into the intercooler, which we do not want. However, Later model engines, with the revised valvecover design (S14) Do not have this problem nearly as bad. I think a real solution here is, instead of running a catch can on the OEM setup, if possible, we should convert our valvecovers to S14 style. this way we can maintain a short OEM tube, and have finer oil control. Another solution that worked well for me for 4 years of daily driving is, cleaning the intake plumbing every 6-8 months. The oil will slowly make it's way to the compressor wheel, building up over time. If you clean it out once in a while, it should not coat the compressor wheel the way it does after 15 years of never being cleaned.

this thing?
http://i184.photobucket.com/albums/x267/spriso/IMG_1555.jpg

Good call pointing it out. But again, like you said, we both agree the signal there is very very weak. Big straw, little straw, which one will respond faster to a change in vacuum signal? This is just a guess but that restrictor may be an effort to IMPROVE the amount of vacuum signal during boost. Like switching to a smaller vacuum line to improve transient response of a signal. It isn't going to hamper FLOW because we both know there is barely any net flow coming out of the valvecover. Even with a big vacuum signal there is still quite a restriction present at the T in the cover.

4. I was also thinking about the EPA, and how they do not exist in Japan. And how every engine manufacturer seems to feed the valvecover gasses back into the engine during wide open throttle. Just something to kick around atm.